The present invention relates to an antenna on implantable medical devices that are suitable for placement in living tissue, and more particularly to small implantable stimulators or sensors, hereafter referred to as microstimulators or microsensors. Such medical devices have electrodes in contact with muscle or nerve fibers, through which the devices electrically stimulate the muscle or nerve fibers, or sense one or more physiological states present in the muscle or nerve fibers. More particularly, the invention relates to an antenna for such implantable microdevices, for both receiving signals from an external device, and transmitting signals to an external device.
It is desired to create a conductive metal line or coating on the small implantable stimulators or sensors that will perform as an antenna. Formation of an electrically conductive metal line on a glass or ceramic body is well known to one skilled in the art. Ceramic literature is replete with examples of metal formation on ceramic or glass where a slurry containing metal powder is applied by painting, silk screening, dipping, brushing the slurry on the ceramic and heat-treating it in traditional methods to leave an electrically conductive line or pattern of lines on the glass or ceramic. Every day examples include electrically heated backlights in automobiles for defogging the rear window when a switch is engaged.
It is also known in the ceramics art to form an antenna in glass or ceramic by placing a metallic conductor, such as a wire, on or within the ceramic body. One well-known example of such an antenna also comes from the automobile arena, where placing a wire between the glass layers of the laminated windshield forms a radio antenna for reception of AM or FM stations.
The need for small implantable stimulators or sensors that transmit or receive signals by means of an antenna arises from a variety of neuromuscular needs, such as neurological disorders that are often caused by neural impulses failing to reach their natural destination in otherwise functional body systems. Local nerves and muscles may function, but, for various reasons, injury, stroke, or other cause, the stimulating signals do not reach their natural destination.
For example, paraplegics and quadriplegics have intact muscles but lack the complete brain-to-muscle nerve link that conducts the signal to the muscles.
Prosthetic devices have been used for some time to provide electrical stimulation to excite muscle, nerve or other cells to provide relief from paralysis, and various other physical disorders have been identified which may be treated by electrical stimulation devices. Some of these devices have been large bulky systems providing electrical pulses through conductors extending through the skin. Disadvantageously, complications, including the possibility of infection, arise in the use of stimulators that have conductors extending through the skin.
Other smaller stimulators have been developed that are fully implantable and that are controlled through high frequency, modulated RF, telemetry signals. Such systems designed to stimulate nerves or muscles to provide motion are know as Functional Electrical Stimulation (FES) systems. An FES system using telemetry signals is set forth in U.S. Pat. No. 4,524,774 for “Apparatus and Method for the Stimulation of a Human Muscle.” The '774 patent teaches a source of electrical energy, modulated in accordance with desired control information, to selectively power and control numerous, small stimulators, disposed at various locations within the body. Thus, for example, a desired progressive muscular motion may be achieved through the successive or simultaneous stimulation of numerous stimulators, directed by a single source of information and energy outside the body.
Many difficulties arise in designing implanted stimulators that are small in size, and in passing sufficient energy and control information to the stimulators to satisfactorily operate them without direct connection. A design of a small functionally suitable stimulator, a microstimulator, is taught is U.S. Pat. No. 5,324,316 for “Implantable Microstimulator.” The '316 patent teaches all the elements required for successful construction and operation of a microstimulator. The microstimulator is capable of receiving and storing sufficient energy to provide the desired stimulating pulses, and is also able to respond to received control information defining pulse duration, current amplitude and shape. The microstimulator of the '316 patent is easily implanted, such as by injection by a hypodermic needle. The '316 patent is incorporated herein by reference.
Known microstimulators utilize a telemetry receiver based on modulating an inductive power signal provided to the microstimulator. Similarly, signals are transmitted from the microstimulator using the same circuits. By using components already present in the microstimulator, these telemetry systems do not require substantial additional circuitry. However, such inductive telemetry methods are limited by the resonant frequencies of the existing coil, which are typically below 2 MHz. While this approach has proven adequate for many applications, there are potential problems with interfering signals. Further, much higher frequencies, 402 to 405 MHz, have been designated by the Federal Communications Commission (FCC) for use with medical devices.
Telemetry methods utilizing monopole and dipole antennas are known for use in the FCC designated frequency range, however, such antennas are, primarily, electrical field devices. Electrical field devices suffer from high tissue detuning (i.e., the surrounding tissue interacts with the electrical nature of circuit components to the extent that some effectiveness of tuning is lost) and may not provide the best performance for implantable devices. Other telemetry systems utilizing a loop antenna inside the microdevice are also known in the art, see U.S. Pat. No. 6,804,561 B2, for example. Loop antennas have the advantage of being magnetic field devices, and are therefore less susceptible to tissue detuning. However, placing the loop antenna inside the case of a microdevice exhausts scarce space within the microdevices.
A need exists for a telemetry system that does not suffer from high tissue detuning loss, that does not take up substantial space inside the implantable microdevice, and that is suitable for operation in the 402 to 405 MHz frequency range.